1. Field of the Invention
The present invention relates to an improved copper alloy having improved properties and the method of manufacture thereof. More particularly, the invention relates to a spray cast copper-iron-zirconium alloy having improved mechanical properties.
2. Background Information
Copper based alloys are widely used for electronic, electrical, and thermal applications. Electrical connectors and leadframes are usually formed from copper alloys to exploit the high electrical conductivity inherent in the alloys. Heat sinks, heat exchanger coils, and cooling fins are also manufactured from copper based alloys to take advantage of the excellent thermal conductivity of the alloys.
The copper based alloys are often cold worked following casting to increase the strength of the alloy. When exposed to elevated temperatures, the alloys recrystallize. Recrystallization is accompanied by a loss of structural strength. This phenomenon is often expressed in terms of softening resistance. Softening resistance is a measure of the ability of an alloy to resist deformation when exposed to elevated temperatures. It is desirable to fashion a copper based alloy having high thermal conductivity and high electrical conductivity which also resists softening at elevated temperatures.
The softening of the alloy is a consequence recrystallization. Introducing a means to inhibit recrystallization is a means to improve sottening resistance.
As disclosed in Chapter 22 entitled "Mechanical Properties of Multiphase Alloys" of Physical Metallurgy by R. W. Cahn et la, a way to inhibit recrystallization is to supply the alloy with a uniform dispersion of dispersoids. These dispersoids should have a mean size of less than about 1 micron. Larger sized dispersoids deform the crystal grains and introduce intense stress ingredients and can reduce the recrystallization temperature.
Several means to inhibit recrystallization are known. For example, the addition of specific alloying elements to the base alloy. The alloying elements precipitate upon exposure to heat and form a second phase. This process is known as precipitation hardening. The second phase is small, typically on the order of nanometers, and forms throughout the alloy. An example of a copper based precipitation hardenable alloy is copper alloy C724 which has a composition of from about 10% to about 15% by weight nickel, from about 1% to about 3% by weight aluminum, up to about 1% by weight manganese, from about 0.05% to less than about 0.5% by weight magnesium, less than about 0.05% by weight silicon and the balance copper. Copper alloy C724 is disclosed in U.S. Pat. No. 4,434,016 to Saleh et al.
Another means to inhibit recrystallization is known as dispersion hardening. For example, by internally oxidizing the alloy so that oxide particles are dispersed through the alloy. Internal oxidation of a copper based alloy has been disclosed in U.S. Pat. No. 3,615,899 to Kimura et al.
Both precipitation hardening and dispersion hardening involve altering the internal structure of the alloy subsequent to solidification. The processes require additional thermal treatments adding to the cost of the alloy. Further, these prior art processes are self-limiting as to the quantity and size of the second phase particulate as well as to the composition of the alloy.
Yet another way to form a second phase within a copper based alloy is through solidification. The second phase may form either at the beginning of solidification, for example a peritectic type reaction or at the end of solidification, for example a eutectic type reaction.
During a conventional casting process such as direct chill casting, the solidification rate is relatively slow. Second phase dispersoids are formed during solidification. The dispersoids increase in size and coarsen during the relatively long time period for solidification. The coarse dispersoids can be deformed and elongated or breakup during cold rolling to form stringers. Stringers adversely affect the properties of the alloy by introducing directionally to the ductility and bend properties. Furthermore, the size and distribution of the second phase precludes grain boundary pinning required to prevent recrystallization.
If the solidification rate of the cast alloy is increased, control over the second phase development is improved. One means to increase the solidification rate is through rapid solidification. Rapid solidification involves depositing a molten stream of metal on a chilled collector surface. To maintain the high chill rate, the alloy is cast as a thin ribbon. The ribbon is not suitable for leadframe or connector applications where cross-sectional thicknesses in the range of from about 5 mils to about 20 mils are required.